The imaging elements to which this invention relates can be of many different types depending on the particular use for which they are intended. Such elements include, for example, photographic, electrophotographic, electrostatographic, photothermographic, migration, electrothermographic, dielectric recording and thermal-dye-transfer imaging elements.
Layers of imaging elements other than the image-forming layer are commonly referred to as auxiliary layers. There are many different types of auxiliary layers such as, for example, subbing layers, backing layers, overcoat layers, receiving layers, stripping layers, antistatic layers, transparent magnetic layers, and the like.
Support materials for an imaging element often employ auxiliary layers comprising glassy, hydrophobic polymers such as polyacrylates, polymethacrylates, polystyrenes, or cellulose esters, for example. One typical application for such an auxiliary layer is as a backing layer to provide resistance to abrasion, scratching, blocking, and ferrotyping. Backing layers may be applied directly onto the support material, applied onto a priming or "subbing" layer, or applied as an overcoat for an underlying layer such as an antistatic layer, transparent magnetic layer, or the like. When the backing layer is an overcoat for an antistatic layer the backing layer may need to protect the antistatic layer from attack by film processing solutions in order to preserve the antistatic properties of the film after image processing. For example, U.S. Pat. No. 4,203,769 describes a vanadium pentoxide-containing antistatic layer that is overcoated with a cellulosic layer.
These glassy polymers are typically coated from organic solvent-based solutions to yield a continuous film upon evaporation of the solvent. However, because of environmental considerations it is desirable to replace organic solvent-based coating formulations with water-based coating formulations. The challenge has been to develop water-based coatings that provide similar or improved physical and chemical properties in the dried film that can be obtained with organic-solvent based coatings.
The problem of controlling static charge is well known in the photographic art. The accumulation and discharge of static charges can lead to a variety of problems such as dirt and dust attraction, coating imperfections such as mottle, and static marks. The accumulation of static charge is related to both the electrical conductivity and the charging characteristics of the imaging element. Incorporation of conductive agents such as salts, semiconductive metal oxide fillers and polymers, for example, in auxiliary layers has been used effectively in the photographic art as a means of increasing the electrical conductivity of an imaging element.
The accumulation of static charge may be reduced by controlling the triboelectric charge generated on the surface of imaging elements as a result of friction and separation of surfaces during the manufacture and use of the imaging element. The incorporation of fluorine-containing surface active agents in the surface layers of imaging elements as a means of controlling triboelectric charge generation has been disclosed, for example, in U.S. Pat. Nos. 3,589,906, 3,666,478, 3,884,609, 3,888,678, 4,330,618, and others. European Patent Application EP 0 693 709 A describes a light sensitive material having at least one hydrophilic colloid layer containing in combination, at least one fluoropolymer and at least one fluorochemical surface active agent. Such layers reportedly improve the antistatic behavior of the light sensitive material. Typical compositions in this patent application comprise gelatin as the hydrophilic colloid, fluorochemical surface active agent, and an aqueous dispersion of fluoropolymer. Such hydrophilic layers would be expected to have poor barrier performance and abrasion resistance.
Frequently, when the auxiliary layer serves as the outermost layer, as is the case for a backing layer, it is desirable for this layer to have a low coefficient of friction (COF) to provide proper conveyance properties and to protect the imaging element from mechanical damage during the manufacturing process or customer use. It is known to protect imaging elements against mechanical damage by coating them with a layer comprising a lubricant such as a wax. Often such a wax overcoat is applied over an abrasion resistant layer. However, the need to apply these two separate layers increases both manufacturing complexity and cost.
While the aforementioned prior art references relate to some aspects of the present invention, they are deficient with regard to simultaneously satisfying all the physical, chemical, and manufacturing requirements for providing an improved auxiliary layer for imaging elements that is applied from aqueous medium. The present invention provides an aqueous coating composition which forms a dried layer with excellent physical and chemical properties such as transparency, abrasion resistance, coefficient of friction, triboelectric charging, and barrier properties.